S. Raunier et al. / Chemical Physics Letters 368 (2003) 594–600
599
decreased while the H-bond distances NH. . .N
with the NH3 solvents increased, continuously
and HNCO. The more typical OCNÀ (2165 cmÀ1
)
and NHþ4 (1495 cmÀ1) IR absorptions are shifted
versus those recorded by the ISO spectra of pro-
tostellar objects.
from NHþ4 ðNH3Þ to NH4þðNH3Þ [28]. In this case,
4
ꢁ
the H-bond distances with all NH3 are 1.920 A.
This is good agreement with our values 1.952,
ꢁ
Thus, the two bands assigned to NHþ4 OCNÀ in
the ISO spectra are generally reported at 2165
cmÀ1 (mOCNÀ) and 1460 cmÀ1 (dNHþ4 ) [17]. We
think that these differences could be explained
from the environment of NHþ4 OCNÀ. In this study
we did not consider H2O, which is the major
constituent of the interstellar grains. NH3/H2O ice
could modify the frequency of the ions, we also
consider studying the behavior of HNCO co-de-
posited with such a mixture. For example, the
mOCNÀ frequency changes from 2151 cmÀ1 in an
NH3 environment to 2170 cmÀ1 in an H2O envi-
ronment. The solid NHþ4 OCNÀ frequencies at
T ¼ 160 K are also very much altered modified
compared to NHþ4 OCNÀ frequencies in an NH3
environment (T < 125 K).
1.892 and 1.945 A for the system (n ¼ 3), where
the symmetry is broken.
Quantum calculations confirm the spontaneous
character of the reaction between HNCO and
NH3 at 10 K. Due to the proton donor character
of the HNCO molecule, we confirm that the
ionization process occurs if one NH3 molecule is
in interaction with HNCO via its lone electron
pair, and if this NH3 is solvated enough. Three
NH3 molecules are at least required to induce the
proton transfer.
5. Discussion and conclusion
We showed that when HNCO is co-deposited
with the NH3 (1/10) mixture, an acid–base reaction
occurs between HNCO and NH3 as soon as 10 K
to produce NHþ4 OCNÀ. The more typical vibra-
tional bands of NH4þOCNÀ, which serve in prob-
ing its identification, at 2151 and 1495 cmÀ1 are in
excellent agreement with the values reported for
NHþ4 OCNÀ induced during the photolysis of CO/
NH3 ices [29]. However, the behavior of HNCO is
different when it is co-deposited with NH3 and
when it is adsorbed on NH3 amorphous films at 10
K. In the latter case, the system is not reactive.
Nevertheless, upon sample warm-up at a temper-
ature of 90 K, NHþ4 OCNÀ can be formed but in a
weak proportion because it is only formed at the
interface of the two solid phases. The NHþ4 OCNÀ
formation can be explained from a dynamical
reconstruction of the NH3 film. The changes ob-
served on the NH3 infrared spectrum at 70 K
suggest a transformation from an amorphous to
metastable phase [20]. This transformation is fa-
vorable to solvation of HNCO by NH3 molecules
and it follows the emergence of a reaction as
confirmed by the quantum calculations.
In the same way, the mNCO frequency of HNCO
changes significantly according to its environment:
from 2259 cmÀ1 in the HNCO/NH3 system to 2242
cmÀ1 in the HNCO/H2O [17] system compared
with the 2252 cmÀ1 expected for HNCO solid at 10
K. We believe that the non-detection of HNCO in
the interstellar grain, nevertheless produced in
laboratory during the photolysis of CO/NH3 or
CO/NH3/H2O mixtures [5], can be due both to its
great reactivity to NH3 and to its fast photode-
composition [30].
Further study concerning HNCO adsorbed on
solid NH3 and HNCO in NH3/H2O ices must be
envisaged.
References
ꢀ
[1] J. Crovisier, D. Bockelee-Morvan, Space Sci. Rev. 90
(1999) 19.
[2] E.F. van Dishoeck, G.A. Blake, Annu. Rev. Astron.
Astrophys. 36 (1998) 317.
[3] B.E. Turner, R. Terzieva, E. Herbst, Astrophys. J. 518
(1999) 699.
[4] S.B. Charnley, P. Ehrenfreund, Y.-J. Kuan, Spectrochim.
Acta Part A 57 (2001) 685.
Some other observations providing useful in-
formation on the identification of the species in the
interstellar medium can be obtained from the po-
sition of the vibrational bands of OCNÀ; NH4þ
[5] D.C.B. Whittet, W.A. Schutte, A.G.G.M. Tielens, A.C.A.
Boogert, T. de-Graauw, P. Ehrenfreund, P.A. Gerakines,
F.P. Helmich, T. Prusti, E.F. van-Dishoeck, Astron.
Astrophys. 315 (1996) L357.